Conventionally, thermal transfer method has been widely used as a simple printing method. Thermal transfer method is an image forming method wherein a thermal transfer sheet which is provided with a colorant layer formed on a surface of a substrate sheet thereof is superposed on a thermal transfer image-receiving sheet which is optionally provided with an image receiving layer, then the back side of the thermal transfer sheet is heated by a heating means such as a thermal head with a shape of an intended image, and thereby the colorant included in the colorant layer is selectively transferred onto the thermal transfer image-receiving sheet to form the image on the thermal transfer image-receiving sheet.
The thermal transfer method may be divided into two methods, i.e., melt-transfer method and sublimation transfer method. The melt-transfer method is an image forming method wherein a thermal transfer sheet, in which a thermally fusible ink layer comprising a colorant, such as a pigment, and a binder for dissolving or dispersing the colorant, such as a thermally fusible wax or resin, is carried on a substrate sheet such as a PET film, is used; then, an energy in accordance with image information is applied to the thermal transfer sheet by a heating means such as a thermal head; and thereby the colorant is transferred onto the thermal transfer image-receiving sheet, such as a plastic sheet or paper, together with the binder. The image formed by the melt-transfer method is suitable for recording binary images such as characters, since the image has excellent sharpness and high concentration.
On the other hand, the sublimation transfer method is another image forming method wherein a thermal transfer sheet, in which a dye layer comprising a dye which is able to be transferred thermally principally by sublimation, and a resin binder for dissolving or dispersing the dye, is carried on a substrate sheet such as a PET film, is used; then, an energy in accordance with image information is applied to the thermal transfer sheet by a heating means such as a thermal head; and thereby only the dye is transferred onto a substrate sheet, such as a plastic sheet or paper (a thermal transfer image-receiving sheet which is optionally provided with a dye receiving layer). With respect to the sublimation transfer method, since the transferring amount of dye can be controlled by the amount of energy applied, it is possible to form a gray-scale image in which image density of every dot of the thermal head is controlled. In addition, since the colorants used are dyes, the image formed has transparency. Thus, when dyes of different colors are superposed, the reproducibility of neutral tints becomes excellent. Therefore, when using thermal transfer sheets of different colors such as yellow, magenta, cyan, black or the like, and transferring dye of each color on the thermal transfer image-receiving sheet so as to superpose the dyes each other, it is possible to form a photographic full-color image of high quality in which reproducibility of neutral tints is excellent.
Along by the development of various hardware and software related to multimedia, this thermal transferring method have been expanding its market as a full-color hard copy system for digital images represented by computer graphics, satellite static images, CD-ROM or the like, and for analog images such as video. The thermal transfer image-receiving sheet according to the thermal transfer method includes a wide range of concrete applications. As typical examples, proof printing; image output; output of plan or design, such as those drawn by CAD/CAM, etc.; output use for various medical analytical or measuring instruments such as CT scan, endoscopic camera, etc.; and a substitute for instant photos; as well as, output of picture of face to an identification paper or ID card, credit card, or other cards; and applications as composite photograph or souvenir picture at an amusement facilities such as amusement park, amusement arcade, museum, and aquarium, etc., can be mentioned.
With the diversification of use of the above-mentioned thermal transfer image-receiving sheet, there is an increasing demand for forming a thermal transfer image on an arbitrary object. As the object for forming the thermal transfer image, a purpose-built thermal transfer image-receiving sheet which is provided with a receiving layer on the substrate is usually utilized. However, in this case, the substrate or the like subjects to some type of constraints. Under these circumstances, Patent literature 1 discloses an intermediate transfer medium in which the receiving layer is provided on the substrate so that the receiving layer can be peeled off from the substrate. According to the intermediate transfer medium, by transferring the dye of the dye layer to the receiving layer so as to form an image, and then heating the intermediate transfer medium, it is possible to transfer the receiving layer onto which the dye has been transferred to an arbitrary transcription receiving article. Thus, it becomes possible to form a thermal transfer image without concern for the constraints about the kind of transcription receiving article.
On the other hand, the thermal transfer image, which is formed by using the intermediate transfer medium mentioned above, suffers with a problem of lack of durability including weather resistance, abrasion resistance, chemical resistance, etc., since the receiving layer onto which an image is formed is positioned on the outermost surface. Then, recently, as shown in Patent literature 2, an intermediate transfer medium in which a release layer, a protective layer, and a layer functioned both as receiving layer and adhesive layer is provided on a substrate has been proposed. According to the intermediate transfer medium, since the protective layer is formed on the surface of the thermal transfer image, it is possible to impart durability to the thermal transfer image.